Drill thread milling cutters of this type are known, for example, from WO 2004/022274 A1 or U.S. Pat. No. 4,651,374 A or U.S. Pat. No. 4,761,844 A or from Handbuch der Gewindetechnik und Frastechnik [Manual of threading practice and milling practice], publisher: EMUGE-FRANKEN, publishing firm: Publicis Corporate Publishing, year of publication: 2004 (ISBN 3-89578-232-7), page 354. With such a tool, it is possible first of all to make a generally cylindrical bore in a workpiece. For this purpose, known drill thread milling cutters have drilling cutting edges with which the bore can be produced. Once this has been done, the cutter is fed in radially, i.e. it is displaced from the centre of the bore into a position parallel to the bore axis. Arranged on the shank of the tool are milling cutting edges or milling teeth, with which a thread can then be made in the bore by means of circular thread milling. In the process, the cutting edges or milling teeth of the cutter engage jointly in a bore wall, the thread being produced by the tool being rotated and by a helical movement of the entire tool being carried out at the same time.
The drill thread milling cutter according to WO 2004/022274 A1 is said to be suitable for steel and to this end is provided with core-hole-producing cutting edges in a drilling region and with thread-producing cutting edges in an axially adjoining thread milling region. In their radial dimensions, the thread-producing cutting edges are 3% smaller than the core-hole-producing cutting edges. The absolute radial difference in distance between the core-hole-producing cutting edges and the thread-producing cutting edges therefore depends on the diameter of the drill thread milling cutter. At a typical diameter of a drill thread milling cutter of about 8 mm, the radial difference in distance between the core-hole-producing cutting edges and the thread-producing cutting edges is thus about 0.24 mm or 240 μm. Two drilling flutes arranged offset from one another by 180° are provided, at the end face of which the core-hole-producing cutting edges or drilling cutting edges are located and adjoining which in axial extension is a first axial row of thread-producing teeth or cutting edges. Furthermore, two smaller milling flutes, offset by about 90° are provided between the two drilling flutes, at which milling flutes in each case additional axial rows of milling cutting edges or milling teeth are located as additional thread-producing cutting edges. The first rows of thread-producing cutting edges at the drilling flutes are set back radially or are lowered relative to the drilling flute, in each case in order to reduce the effect of the drilling chip, which is removed by the drilling flute, on these thread-producing cutting edges at the drilling flute. The flutes become smaller from the tip of the tool towards the drill shank, as a result of which the drill thread milling cutter is strengthened in its lateral rigidity. The core diameter at the flutes is thus increased from the tip towards the shank.
Publications U.S. Pat. No. 4,651,374 A and U.S. Pat. No. 4,761,844 A concurrently describe (FIGS. 1 to 4) a drill thread milling cutter having four flutes which are offset from one another by 90° and whose cross-sections and volumes are the same, the cross-section and volume being identical axially over their entire length relative to the tool axis. At two flutes offset from one another by 180°, end drilling cutting edges are provided at the tip of the tool, these end drilling cutting edges tapering to a drill point, and, axially adjoining the end drilling cutting edges, thread milling cutting edges in the form of tooth rows are arranged at these drilling flutes. No drilling cutting edges are provided at the other two flutes, but rather only thread milling cutting edges designed as axial tooth rows. These further flutes therefore merely serve as milling flutes. The milling teeth are formed on annular segments arranged perpendicularly to the tool axis. Running centrally through the tool shank and the thread milling region is a coolant passage, which branches in the drilling region into four sectional passages which open out at the drill point in the tooth webs at which the thread milling teeth are located. The thread milling region with its thread milling cutting edges or thread milling teeth has a uniform maximum diameter which is equal to the maximum diameter of the drilling region, so that the thread milling region can be directed without interference into the bore which the drilling region has produced.
In addition to U.S. Pat. No. 4,651,374 A, U.S. Pat. No. 4,761,844 A discloses further embodiments of drill thread milling cutters (FIG. 5 to FIG. 12), in which only two flutes are provided, at which both drilling cutting edges and, following them, thread milling cutting edges are located as tooth rows. In this case, in addition to a drilling region for producing the bores, an end milling region and also a drilling region having a central recess are also disclosed. The thread milling cutting edges of the thread milling region again have a maximum diameter which is equal to or not greater than the maximum diameter of the bore-producing region, so that the thread milling region can be inserted without interference into the bore which has been produced by the bore-producing region. In addition, a countersinking bevel is provided between the thread milling region and the shank. The two flutes in this further embodiment also have the same cross-section and the same volume over the entire length of the working region.
In the drill thread milling cutter according to WO 2004/022274 A1, the angular pitches between milling cutting edges following one another in the direction of rotation amount to an angle greater than 90° between a milling cutting edge at the drilling flute and a milling cutting edge preceding in the direction of rotation, this angle being around 103° according to the drawing, and correspondingly to the complementary angle of 77° at the milling cutting edge following in the direction of rotation. In the drill thread milling cutter according to U.S. Pat. No. 4,651,374 A and U.S. Pat. No. 4,761,844 A, the angular pitches are constant and equal to 90° or 180° and thus an identical pitch is realized.
Whereas drill thread milling cutters of the previously known type can usually readily work short-chipping material, such as grey cast iron and brass for example, problems arise if long-chipping material, such as steel for example, is to be machined. This is because the long chips get into the thread milling region of the tool during the drilling, so that said thread milling regions may be damaged or destroyed.
Furthermore, it has been found in tests carried out by the applicant that the previously known drill thread milling cutters of the type mentioned at the beginning are relatively susceptible to fracture, in particular when long-chipping and/or high-strength material such as steel is machined.
In addition, the known drill thread milling cutters, as long as they are ready for operation, have a relatively low cutting capacity.